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triathlon Physical exercise is any bodily activity that enhances or maintains physical fitness and overall health or wellness. It is performed for various reasons. These include strengthening muscles and the cardiovascular system, honing athletic skills, weight loss or maintenance and for enjoyment. Frequent and regular physical exercise boosts the immune system, and helps prevent the "diseases of affluence" such as heart disease, cardiovascular disease, Type 2 diabetes and obesity.12 It also improves mental health, helps prevent depression, helps to promote or maintain positive self-esteem, and can even augment an individua(disambiguation). EnlargeCompetitor emerging from the swim portion of a triathlonl's sex appeal or body image, which again is also linked with higher levels of self-esteem.3 Childhood obesity is a growing global concern4 and physical exercise may help decrease the effects of childhood obesity in developed countries Physical exercise is important for maintaining physical fitness and can contribute positively to maintaining a healthy weight, building and maintaining healthy bone density, muscle strength, and joint mobility, promoting physiological well-being, reducing surgical risks, and strengthening the immune system. Exercise also reduces levels of cortisol.[citation needed] Cortisol is a stress hormone that builds fat in the abdominal region, making weight loss difficult.[citation needed] Cortisol causes many health problems, both physical and mental.9 Frequent and regular aerobic exercise has been shown to help prevent or treat serious and life-threatening chronic conditions such as high blood pressure, obesity, heart disease, Type 2 diabetes, insomnia, and depression.10 Endurance exercise before meals lowers blood glucose more than the same exercise after meals.11 According to the World Health Organization, lack of physical activity contributes to approximately 17% of heart disease and diabetes, 12% of falls in the elderly, and 10% of breast cancer and colon cancer.12 There is some evidence that vigorous exercise (90–95% of VO2 Max) is more beneficial than moderate exercise (40 to 70% of VO2 Max).13 Some studies have shown that vigorous exercise executed by healthy individuals can increase opioid peptides (a.k.a. endorphins, naturally occurring opioids that in conjunction with other neurotransmitters are responsible for exercise-induced euphoria and have been shown to be addictive), increase testosterone and growth hormone,14 effects that are not as fully realized with moderate exercise. More recent research1516 indicates that anandamide may play a greater role than endorphins in "runner's high". Both aerobic and anaerobic exercise also work to increase the mechanical efficiency of the heart by increasing cardiac volume (aerobic exercise), or myocardial thickness (strength training). Such changes are generally beneficial and healthy if they occur in response to exercise. Not everyone benefits equally from exercise. There is tremendous variation in individual response to training: where most people will see a moderate increase in endurance from aerobic exercise, some individuals will as much as double their oxygen uptake, while others can never augment endurance.1718 Similarly, only a minority of people will show significant muscle growth after prolonged weight training, while a larger fraction experience improvements in strength.19 This genetic variation in improvement from training is one of the key physiological differences between elite athletes and the larger population.2021 Studies have sh EnlargeUS Marines exercising on the [[USS Bataan (LHD-5)|USS Bataan]]own that exercising in middle age leads to better physical ability later in life.22